Shares in the company shot up on the news and were trading 8.9% to 2.94 pence in London by 1:16 pm local time.

It's the largest found so far this year and follows the recovery of a 46-carat, gem quality white rock in December, which sold for over $1 million.

Firestone said the diamond was recovered in the northern, low grade part of the pit where the bulk of its mining activities will take place in the coming months.

"Although the market for the smaller stones has been under pressure, we've seen continued demand and good prices realised for special stones," chief executive Paul Bosma said in the statement.

Firestone spent $185 million building Liqhobong, which started production in late 2016, and boasts over 11 million carats in reserve. The total open pit resource contains over 17 million carats down to 393 metres.

Last week, the company said an equipment failure in the mine's plant in January was expected to impact tonnes treated and carats recovered in the March quarter. However, it stuck to its guidance for the full-year of 820,000 to 870,000 carats.